A damning Ofsted report has sent “outstanding” Highbury Grove School tumbling into special measures, meaning it will be forced to become an academy.

A paper, published today, gave the school an “inadequate” rating: the lowest on the scale of four.

City of London Academy Trust, which runs Mount Carmel RC School in Archway and City of London Academy Islington, is understood to be in talks about taking the reins.

The 1,200-pupil secondary school in Highbury New Park was left without a permanent skipper following headteacher Tom Sherrington’s shock departure on January 20 – less than a fortnight before the publication of the report.

Ofsted had awarded Highbury Grove top marks in 2010 under Mr Sherrington’s predecessor Truda White – but inspectors who visited at the end of the autumn term painted a different picture.

In the paper, lead investigator Helen Matthews tells of “poor academic progress hindering many pupils from moving on to the next stage of education and training” and “leaders having an over-generous view of the quality of education they are providing”.

She writes: “Progress for all pupils in 2015 and 2016 was significantly below the progress made by pupils nationally. School leaders do not have an accurate understanding of the quality of education they are providing. Leaders do not base their assessment of the school’s effectiveness on solid evidence because their monitoring processes are not rigorous.

“Leaders have failed to improve attendance. Too many pupils are persistently absent from school and this has a detrimental impact on the progress they are making.”

The report also rounded on governors for not challenging Mr Sherrington’s decisions: “They have often been too accepting of information and evidence presented to them, particularly in relation to teachers’ salary and pay progression.”

He came under scrutiny last year after the Gazette revealed the school’s finances had become precarious. Its bank balance plunged from £600,000 in the black to £700,000 in debt in a year and a half. Mr Sherrington told this newspaper cash reserves were being used to protect jobs in the face of falling government funding.

However the report states that the school is now “financially more stable” following a staffing restructure.

Ms Matthews also said “some pupils’ behaviour around the main school site puts other pupils at risk”. She added: “Incidents of bullying are frequent. Staff and pupils voiced their concern about their own safety. Inspectors witnessed conduct that put others at risk during the inspection. Records kept by the school indicate that these incidents are too common.”

In a letter to parents sent out on Saturday, the school, which is being led by an acting headteacher – Aimee Lyall – said: “We know that you will share our disappointment at the result. However, the school has been working very hard in the past weeks to remediate the failings outlined in the report.

“We are fortunate to have an excellent teaching staff at HGS and our priority now is to support their commitment in every way possible. Our increased focus on behaviour will be key, as will the support of all school leaders to ensure teaching and learning at the school is of the highest possible standards.”

It added: “We are making some changes to the school day and to the way the behaviour policy works that we think will improve behaviour substantially. Starting after half-term, lunch times will be staggered, so only about half of the school will be at lunch at one time. This should help with the incidence of poor behaviour at that time.”

The result puts the town hall in the embarrassing position of having to relinquish control of what had been a flagship school.

Cllr Joe Caluori, executive member for children, young people and families, said: “We want every child in Islington to get the best possible education.

“We will continue to work closely with Highbury Grove School and provide the necessary support towards achieving this goal so that pupils, parents and staff can be confident in its future.”

Meanwhile Mr Sherrington, who styled himself as the “head guru teacher” on Twitter and his blog posts, has now launched a consultancy business.

In 2013, 18 months before he took over as head at Highbury Grove, he tweeted: “If there was no Ofsted, no tables, no SLT – just you and your class – what would you choose to do to make it great? Do that anyway.”